162 THE VOYAGE OF H.M.S. CHALLENGER. 



being irregular and irregularly interrupted. The centre of the valve is for the most part 

 smooth and bears only a small group of granules. Although it is not possible to deter- 

 mine the cause of the polygonal form it may be conjectured that it has resulted from the 

 detachment of its outer border. The generic and specific determinations which have been 

 given are to be looked upon as provisional — (l.) because the surface of the valve, like 

 those of several of the species above enumerated, is very delicately striated, and (2.) because 

 the outline here presented is probably abnormal. 



Coscinodiscus megacoccus, n. sp. (Plate XVII. fig. 2.) 



E minimis; cellulis vel areolis subhexagonalibus arrandiusculis; mar seine cellulis 

 subquadratis constituto. In Oceano Pacifico. 



This very minute organism from the Pacific Ocean is singular in being covered by a 

 limited number of large cellules and by possessing a distinct border of cellules or sub- 

 quadrate granules. In these respects it presents affinities with the similarly ornamented 

 Coscinodiscus subconcavus, Grun., which is figured in Schmidt's Atlas, plate lxii. fig. 7. 

 It may, however, be readily distinguished from the latter — (1.) by its much smaller 

 diameter, and (2.) by the fact that the cellules are notably larger than in any other 

 known species. 



Coscinodiscus (?) rudis, n. sp. (Plate XXII. fig. 4.) 



Forma distinctis granulis aequalibus elevatioribus grandiusculis instructa, rudem super- 

 ficiem efficientibus. Ad insulas Philippinas. 



The generic determination of this disc, from the Philippine Islands, must be looked 

 upon as provisional, as it may ultimately be found to belong to the genus Pyxidicula, 

 Ehrenb., its valve being sufficiently convex to entitle us to express such a conviction. It 

 is distinguished by its large and distinctly elevated papillae or granules, which are hexa- 

 gonal at the base, but orbicular at the summit. On account of these papillae the external 

 surface, when seen from the zonal aspect, is tuberculated in appearance — a circumstance 

 which has suggested the specific name. 



Coscinodiscus" (?) venulosus, n. sp. (Plate XVII. fig. 1.) 



Forma minima, centro lineolis sinuosis radiantibus distincto. Ad meridiem insulae 

 Kerguelen. 



This small hyaline disc was observed in a collection made to the south of Kerguelen. 

 It only shows a corona of tortuous and ill-defined lines around the centre, where a smooth 

 irregularly bordered area occurs. Although I have provisionally placed this curious valve 

 among the Coscinodisci, its generic determination is by no means free from doubt. 



